1. Field of the Invention
The present invention relates generally to echo cancellars for two-wire-four-wire hybrid circuit, and more specifically to a voice activity detector for controlling the estimation process of an echo canceller under varying talking modes.
2. Description of the Related Art
Echo cancellers are used in transmission systems where a two-wire channel is coupled to a four-wire channel by a hybrid transformer for cancelling undesired waveforms, or echo introduced to the desired signal by the transhybrid coupling resulting from impedance imbalance between these channels. For echo cancellation, a replica of echo is estimated and subtracted from a signal in the transmit (go) path. A signal in the receive (return) path of the four-wire circuit is used for estimating the echo replica and this is only achieved during the absence of a signal in the transmit path of the four-wire circuit since it obstructs the estimation process. A voice activity detector is usually provided to bring the estimation process to a halt whenever the presence of a signal is sensed in the transmit path.
Since there is a singing margin of 6 dB or greater in telephone exchange lines, the signal level at the transmit port path of the four-wire circuit is lower than the signal level at the receive path by more than 6 dB in the absence of a signal in the transmit path. When voice activities exist simultaneously in both of these paths, they are brought to an equal signal level. The prior art voice activity detector has two input terminals for receiving the near-end, transmit signal T and the far-end, receive signal R and operates by sensing the presence of the T and R signals to inhibit the estimation process of the echo canceller.
However, if the echo return loss (ERL) is 10 dB and the level of receive signal R is -10 dBm0, for example, the level of echo is -20 dBm0. If echo estimation is performed in the simultaneous presence of T and R signals and the level of transmit signal T is -20 dBm0 in the absence of an echo, the actual signal level at the input of the voice activity detector will be about -17 dBm0 due to the presence of an echo. Because of the 3-dB enhancement: compared to the echo level of -20 dBm 0 , the apparent echo return loss is 7 dB. If the echo return loss is equal to or more than 6 dB and this value is designed into the voice activity detector, the simultaneous presence of T/R signals would not be precisely detected.